1. Field of the Invention
The present invention relates to a chemically processed steel sheet remarkably improved in corrosion resistance by generation of a converted layer with a self-repairing faculty on a surface of a zinc plating layer.
2. Description of Related Art
Zn or its alloy-coated steel sheets (hereinafter referred to as xe2x80x9czinc-coated steel sheetxe2x80x9d) have been used as corrosion-resistant material. But, when the zinc-coated steel sheet is held as such in a humid atmosphere, exhaust gas or an environment subjected to dispersion of sea salt grains for a long time, its external appearance is worsened due to generation of white rust on the plating layer. Generation of white rust is conventionally inhibited by chromating.
A conventional chromate layer is composed of complex oxides and hydroxides of trivalent and hexavalent Cr. Scarcely-soluble compounds of Cr(III) such as Cr2O3 act as a barrier against a corrosive atmosphere and protects a steel base from corroding reaction. Compounds of Cr(VI) are dissolved as oxoatic anions such as Cr2O72xe2x88x92 from the converted layer and re-precipitated as scarcely-soluble compounds of Cr(III) due to reducing reaction with exposed parts of a steel base formed by working or machining. Re-precipitation of Cr(III) compounds automatically repairs defective parts of the converted layer, so that corrosion-preventing faculty of the converted layer is still maintained after working or machining.
Although chromating effectively inhibits generation of white rust, it obliges a big load on post-treatment of Cr ion-containing waste fluid. In this consequence, various methods using chemical liquors, which contains titanium compound, zirconate, molybdate or phosphate instead of chromate, have been proposed for generation of Cr-free converted layers.
As for generation of a molybdate layer, JP 51-2419 B1 proposed a method of dipping a steel member in a chemical liquor containing magnesium or calcium molybdate, and JP 6-146003 A1 proposed a method of applying a chemical liquor, which contains a partially reduced oxide of Mo(VI) at a ratio of Mo(VI)/total Mo to 0.2-0.8, to a steel member. As for generation of a titanium-containing layer, JP 11-61431 A1 proposed a method of applying a chemical liquor, which contains titanium sulfate and phosphoric acid, to a galvanized steel sheet.
These converted layers, which have been proposed instead of the conventional chromate layer, do not exhibit such a self-repairing faculty as the chromate layer.
For instance, a titanium-containing layer does not exhibits a self-repairing faculty due to insolubility, although it is uniformly generated on a surface of a steel base in the same way as the chromate layer. As a result, the titanium-containing layer is ineffective for suppression of corrosion starting at defective parts formed during chemical conversion or plastic deformation. The other Cr-free converted layers are also insufficient for corrosion prevention due to poor self-repairing faculty.
A chemical liquor, which is prepared by mixing phosphoric acid to an aqueous titanium sulfate solution, is easy to generate precipitates. Once precipitates are generated, it is difficult to uniformly spread the chemical liquor to a surface of a steel base, resulting in generation of an ununiform converted layer. When precipitates are included in the converted layer, adhesiveness of the converted layer and external appearance of the processed steel sheet are worsened. Corrosion resistance of the converted layer would be degraded due to residual sulfate radical. Moreover, composition of the chemical liquor is often varied to a state unsuitable for generation of a converted layer with high quality due to the precipitation.
A manganese-containing converted layer, which is generated from a phosphate liquor, is relatively soluble, and dissolution of the converted layer occurs in a humid atmosphere. In this regard, an effect of the converted layer on corrosion resistance is inferior, even if the converted layer is thickened. Furthermore, the phosphate liquor shall be intensively acidified due to poor solubility of manganese phosphate. The acidified liquor violently reacts with a zinc plating layer, and loses its validity in a short while.
The present invention aims at provision of a processed zinc-coated steel sheet remarkably improved in corrosion resistance by generation of a converted layer, which contains insoluble or scarcely-soluble compounds useful as a barrier for insulation of a steel base from an atmosphere and soluble compounds with a self-repairing faculty for repairing damaged parts of the converted layer.
The present invention proposed a new processed zinc-coated steel sheet comprising a steel base coated with a Zn or its alloy plating layer and a chemically converted layer, which contains at least one complex compound of Ti and Mn, generated on a surface of the plating layer. The complex compound is selected from oxides, phosphates, fluorides and organic acid salts of Mn and Ti. The organic acid salts preferably have carboxylic groups.
A chemical liquor for generation of such a converted layer contains one or more of manganese compounds, titanium compounds, phosphoric acid or phosphates, fluorides and organic acids. The organic acid preferably has a carboxylic group. The chemical liquor is adjusted at pH 1-6.
The present invention proposes another new processed steel sheet comprising the same steel base and a converted layer, which contains both of at least one oxide or hydroxide and at least one fluoride of valve metals, generated on a surface of a Zn or its alloy plating layer. The valve metal is an element, whose oxide exhibits high insulation resistance, such as Ti, Zr, Hf, V, Nb, Ta, Mo or W. A self-repairing faculty of the converted layer is apparently noted by incorporation of a fluoride in the converted layer at an F/O atomic ratio not less than 1/100.
The converted layer may further contains one or more of soluble or scarcely-soluble metal phosphates or complex phosphates. The soluble metal phosphate or complex phosphate may be a salt of alkali metal, alkaline earth metal or Mn. The scarcely-soluble metal phosphate or complex phosphate may be a salt of Al, Ti, Zr, Hf or Zn.
After the chemical liquor is spread to a zinc-coated steel sheet, the steel sheet is dried as such at 50-200xc2x0 C. without washing to generate a converted layer on a surface of a plating layer.
Manganese compounds and valve metal fluorides are effective components other than chromium compound, which give a self-repairing faculty to a converted layer, since these compounds are once dissolved to water in an atmosphere and then re-precipitated as scarcely-soluble compounds at defective parts of the converted layer.
A manganese compound present in a converted layer is partially changed to a soluble component effective for realization of a self-repairing faculty. Accounting the feature of the manganese-containing converted layer, the inventors experimentally added various kinds of chemicals and researched effects of the chemicals on corrosion resistance. In the course of researches, the inventors discovered that addition of titanium compound to a chemical liquor for generation of a manganese compound converted layer effectively suppresses dissolution of the converted layer without weakening a self-repairing faculty.
Improvement of corrosion resistance by addition of titanium compound is supposed by the following reasons, and confirmed by the under-mentioned examples.
A converted layer, which is generated from a manganese phosphate liquor on a surface of a zinc plating layer, is relatively porous. The porous layer allows permeation of corrosive components therethrough to a steel base, resulting in occurrence of corrosion.
On the other hand, when a converted layer is generated from a titanium-containing chemical liquor, pores of the converted layer are filled with titanium compounds precipitated from the chemical liquor. The titanium compounds are insoluble or scarcely-soluble and act as a barrier for shielding a steel base from an atmosphere. Moreover, since the chemical liquor is controlled in an acid range to dissolve the titanium salt, dissolution of Zn from a Zn or its alloy plating layer is promoted. The dissolved Zn is re-precipitated as zinc hydrate useful as a corrosion inhibitor at pores of the converted layer. Consequently, the converted layer is superior of corrosion resistance and exhibits a self-repairing faculty. Furthermore, the titanium compound can be dissolved without excessively falling a pH value, due to co-presence of titanium ion with manganese ion in the chemical liquor.
A valve metal fluoride present in a converted layer is also a soluble component effective for realization of a self-repairing faculty. The valve metal is an element, whose oxide exhibits high insulation resistance, such as Ti, Zr, Hf, V, Nb, Ta, Mo and W. In a converted layer, which contains one or more oxides or hydroxides of valve metals together with one or more fluorides of valve metals, generated on a surface of a zinc plating layer, the oxide or hydroxide acts as a resistance against transfer of electrons and suppresses reducing reaction caused by oxygen dissolved in water (oxidizing reaction of a steel base, in turn), while the fluoride is once dissolved to water in an atmosphere and then re-precipitated as scarcely-soluble compounds at the defective parts of the converted layer. Consequently, dissolution (corrosion) of metal components from a steel base is inhibited. Especially, tetravalent compounds of Group-IV A metals such as Ti, Zr and Hf are stable components for generation of converted layers excellent in corrosion resistance.
The oxide or hydroxide of the valve metal is effective as a resistance against transfer of electrons, when a converted layer is uniformly generated on a surface of a steel base. However, occurrence of defective parts in a converted layer is practically unavoidable during chemical conversion, press-working or machining. At the defective parts where the steel base is exposed to an atmosphere, the converted layer does not sufficiently inhibit corroding reaction. Such the defective parts are automatically repaired by the self-repairing faculty of the valve metal fluoride, and the corrosion-preventing function of the converted layer is recovered.
For instance, a titanium-containing layer generated on a surface of a steel base is composed of TiO2 and Ti(OH)2. When the titanium-containing layer is microscopically observed, defects such as pinholes and very thin parts are detected in the titanium-containing layer. The defects act as starting points for corroding reaction, since the steel base is exposed to an atmosphere through the defects. Although a conventional chromate layer exhibits a self-repairing faculty due to re-precipitation of a scarcely-soluble Cr(III) compound at defective parts, such the self-repairing faculty is not expected as for the titanium-containing layer. Defective parts of the converted layer are reduced by thickening the converted layer, but the hard titanium-containing layer poor of ductility does not follow to elongation of a steel base during working the chemically processed steel sheet. As a result, defects such as cracks and scratches easily occur in the converted layer during working or machining.
On the other hand, co-presence of a fluoride such as XnTiF6 (X is an alkali metal, an alkaline earth metal or NH4, and n is 1 or 2) or TiF4 in the converted layer promotes dissolution of a fluoride to water in an atmosphere and re-precipitation of a scarcely-soluble oxide or hydroxide according to the formula of TiF62xe2x88x92+4H2Oxe2x86x92Ti(OH)4+6Fxe2x88x92. The re-precipitation means realization of a self-repairing faculty. A metal part of the fluoride may be either the same as or different from a metal part of the oxide or hydroxide. Some oxoates of Mo or W useful as a valve metal exhibit such the self-repairing faculty due to solubility, so as to relax restrictions on a kind of a fluoride to be incorporated in a converted layer.
A steel base, which is to be chemically processed according to the present invention, is a steel sheet coated with a Zn or its alloy plating layer by electroplating, hot-dip coating or vacuum deposition coating. The Zn alloy plating layer may be Znxe2x80x94Al, Znxe2x80x94Mg, Znxe2x80x94Ni or Znxe2x80x94Alxe2x80x94Mg. An alloyed zinc-coated steel sheet, which has been subjected to alloying treatment after hot-dip coating, is also used as a steel base for chemical processing.
A chemical liquor for generation of a converted layer containing a complex compound of Mn and Ti is an acid solution containing one or more of manganese compounds and titanium compounds. The manganese compound may be one or more of Mn(H2PO4)2, MnCO3, Mn(NO3)2, Mn(OH)2, MnSO4, MnCl2 and Mn(C2H3O2)2. The titanium compound may be one or more of K2TiF6, TiOSO4, (NH4)2TiF6, K2[TiO(COO)2], TiCl4 and Ti(OH)4.
The manganese compound is preferably added to a chemical liquor at a ratio not less than 0.1 g/l calculated as Mn, to gain a deposition rate of Mn sufficient for corrosion resistance. But, excessive addition of Mn more than 100 g/l unfavorably worsens stability of the chemical liquor. The titanium compound is preferably added at a Ti/Mn mole ratio not less than 0.05 for improvement of corrosion resistance without degrading a self-repairing faculty of the converted layer. An effect of titanium compound on corrosion resistance is intensified as increase of a Ti/Mn mole ratio, but an excessive Ti/Mn mole ratio more than 2 causes instability of the chemical liquor and also rising of a process cost.
The chemical liquor contains phosphoric acid or phosphate, which etches a surface of a Zn or its alloy plating layer to an activated state and changes to a scarcely-soluble phosphate effective for corrosion resistance. The phosphate may be manganese phosphate, sodium dihydrogenphosphate, disodium hydrogenphosphate, magnesium phosphate and ammonium dihydrogenphosphate. Phosphoric acid or phosphate is preferably added to the chemical liquor at a P/Mn mole ratio of 0.2-4. An effect of phosphoric acid or phosphate on corrosion resistance is apparently noted at a P/Mn mole ratio not less than 0.2, but an excessive P/Mn mole ratio above 4 means too-intensified etching action and instability of the chemical liquor.
The chemical liquor further contains one or more of fluorides, which also etch a surface of a Zn or its alloy plating layer to an activated state, and chelate manganese compounds and titanium compounds. The fluoride may be hydrogen fluoride, titanium fluoride, ammonium fluoride, potassium fluoride or silicofluoric acid.
An organic acid with chelating function is further added to the chemical liquor, to maintain scarcely-soluble metals such as Mn and Ti as stable metal ions. The organic acid may be one or more of tartaric, tannic, citric, oxalic, malonic, lactic and acetic acids. The organic acid is preferably added at an organic acid/Mn mole ratio of 0.05-1. An effect of the organic acid on chelation of metal ions for stabilization of the chemical liquor is typically noted at an organic acid/Mn mole ratio not less than 0.05, but an excessive ratio more than 1 falls a pH value of the chemical liquor and worsens continuous processability.
The manganese compound, the titanium compound, the phosphoric acid or phosphate, the fluoride and the organic acid are mixed together at ratios to adjust a pH value of the chemical liquor to 1-6. As falling of a pH value, etching action of the chemical liquor on a surface of the Zn or its alloy plating layer is accelerated, and the surface of the plating layer is reformed to an activated state in a short time. However, excessive falling of a pH value below 1 causes violent dissolution of Zn from the plating layer and instability of the chemical liquor, and an excessively higher pH value above 6 also degrades stability of the chemical liquor due to precipitation of titanium compounds.
A chemical liquor for generation of a converted layer containing compounds of a valve metal is either a coat-type or reaction-type. The reaction-type chemical liquor is preferably adjusted to a relatively low pH value to assure its stability. The following explanation uses Ti as a valve metal, but the other valve metals are also useful in the same way.
A chemical liquor contains a soluble halide or oxoate as a Ti source. Titanium fluoride is useful as both Ti and F sources, but a soluble fluoride such as (NH4)F may be supplementarily added to the chemical liquor. In concrete, the Ti source may be XnTiF6 (X is an alkali or alkaline earth metal, n is 1 or 2), K2[TiO(COO)2], (NH4)2TiF6, TiCl4, TiOSO4, Ti(SO4)2 or Ti(OH)4. Ratios of these fluorides are determined such that a converted layer having predetermined composition of oxide(s) or hydroxide(s) and fluoride(s) is generated by drying and baking a steel sheet after application of the chemical liquor.
An organic acid with chelating faculty may be further added to the chemical liquor, in order to maintain a Ti source as a stable ion in the chemical liquor. Such the organic acid may be one or more of tartaric, tannic, citric, oxalic, malonic, lactic and acetic acids. Especially, oxycarboxylic acids such as tartaric acid and polyhydric phenols such as tannic are advantageous in stability of the chemical liquor, assist a self-repairing faculty of a fluoride and adhesiveness of a paint film. The organic acid is preferably added to the
Orthophosphates or polyphosphates of various metals may be added for incorporation of soluble or scarcely-soluble metal phosphates or complex phosphates in a converted layer.
A soluble metal phosphate or complex phosphate is dissolved from a converted layer, reacted with Zn and Al in a steel base through defective parts of the converted layer and re-precipitated as scarcely-soluble phosphates which assist a self-repairing faculty of a titanium fluoride. An atmosphere is slightly acidified on dissociation of the soluble phosphate, so as to accelerate hydrolysis of the titanium fluoride, in other words generation of scarcely-soluble titanium oxide or hydroxide. A metal component capable of generating a soluble phosphate or complex phosphate is an alkali metal, an alkaline earth metal, Mn and so on. These metals are added as metal phosphates alone or together with phosphoric acid, polyphosphoric acid or another phosphate to the chemical liquor.
A scarcely-soluble metal phosphate or complex phosphate is dispersed in a converted layer, resulting in elimination of defects and increase of strength. A metal component capable of generating a scarcely-soluble phosphate or complex phosphate is Al, Ti, Zr, Hf, Zn and so on. These metals are added as metal phosphates alone or together with phosphoric acid, polyphosphoric acid or another phosphate to the chemical liquor.
Among various kinds of zinc-coated steel sheets, a steel sheet coated with an Al-containing plating layer has the disadvantage that its surface is easily blackened. Such blackening is inhibited by incorporation of one or more salts of Fe, Co and Ni in the converted layer. A self-repairing faculty derived from fluoride and phosphate is sometimes insufficient, when big cracks are generated in the converted layer by plastic deformation of the steel sheet with a heavy work ratio. In this case, the self-repairing faculty is intensified by adding one or more of soluble oxoates of Mo(VI) and W(VI) to the converted layer at a great ratio. Such the oxoates exhibit the same function as Cr(VI) to repair the defective parts of the converted layer, resulting in recovery of corrosion resistance.
One or more lubricants are optionally added to the chemical liquor, to bestow a converted layer with lubricity. The lubricant may be powdery synthetic resins, for instance polyolefin resins such as fluorocarbon polymer, polyethylene and polypropylene, styrene resins such as ABS and polystyrene, or halide resins such as vinyl chloride and vinylidene chloride. An inorganic substance such as silica, molybdenum disulfide, graphite and talc may be also used as the lubricant. Improvement of workability of a processed steel sheet is noted by addition of the lubricant to the converted layer at a ratio not less than 1 mass %, but excessive addition above 25 mass % impedes generation of the converted layer, resulting in degradation of corrosion resistance.
After the chemical liquor prepared as above-mentioned is spread to a Zn or its alloy plating layer formed on a steel sheet by an applicator roll, a spinner, a sprayer or the like, the steel sheet is dried as such without washing to generate a converted layer good of corrosion resistance on a surface of the plating layer. The chemical liquor is preferably applied at a ratio not less than 10 mg/m2 calculated as deposited Mn or at a ratio not less than 1 mg/m2 calculated as deposited valve metal for realization of sufficient corrosion resistance.
Concentrations of elements incorporated in the converted layer are measured by X-ray fluorescence, ESCA or the like. A quantitative effect of the chemical liquor on corrosion resistance is saturated at 1000 mg/m2 calculated as deposited Mn, and further improvement of corrosion resistance is not expected any more even by thickening the converted layer.
As for a converted layer containing valve metal compounds, a corrosion resistance of the converted layer can be evaluated in relation with an F/O atomic ratio, which is calculated from the measured F and O concentrations, on corrosion resistance. Corroding reaction, which starts at defective parts of the converted layer, is remarkably suppressed at an F/O atomic ratio not less than 1/100. Suppression of corrosion proves realization of a self-repairing faculty derived from titanium fluoride incorporated in the converted layer at a quantitatively sufficient ratio.
The steel sheet, which has a converted layer generated from the chemical liquor applied to a surface of a plating layer, may be dried at an ordinary temperature, but is preferably dried in a short time at a temperature of 50xc2x0 C. or higher accounting continuous processability. However, drying at a too-higher temperature above 200xc2x0 C. causes thermal decomposition of organisms of a converted layer, resulting in degradation of corrosion-resistance.
An organic paint film good of corrosion resistance may be laid on the converted layer. Such the paint film is formed by applying a resin paint containing one or more of olefinic resins such as urethane, epoxy, polyethylene, polypropylene and ethylene-acrylic copolymer, styrenic resins such as polystyrene, polyesters, acrylic resins or these copolymers or degenerated resins. The resin paint may be applied to the converted layer by an applicator roll or electrostatic atomization. When a paint film of 0.5-5 xcexcm in thickness is laid on the converted layer, the converted layer surpasses a conventional chromate layer in corrosion resistance. The converted layer can be bestowed with lubricity or weldability by laminating an organic paint film good of electric conductivity thereon.